Design Patterns (and anti-patterns) are techniques of programming. There are a lot of design patterns. It is upto the programmer to pick and choose which patterns to use based on their needs. Singleton is also a design pattern. A singleton is a type of class which can have only 1 object of its kind through the lifecycle of an application. It is very suitable for database, caching, etc. I will use a database class as an example as it is probably the most-widely used class in web development.

Why use a singleton you ask? A popular practice among PHP4’s developers is to make a database object global then use that variable anything a query needs to be run. It works in small applications and even in not-so-big applications. In enterprise applications where you use hundreds (if not thousands) of variables, there is a very good chance that you database object will get mutated or overridden. You also don’t want to have multiple instance of a database class (unless you are running a multi-threaded application).

A database class should be able to connect to a database, run queries, return results and close the connection to the database when it’s done. Keeping that in mind, let’s start writing our database class.

So, we wrote a class called database and made its constructor private so you can’t instantiate it using the “new” keyword. We also wrote a method/function for this class called getInstance() which will return a database object. getInstance() has a local static variable called $obj. It checks if this variable is null. If it’s null then that means there is no object of the type “Database”. So, it creates an object and assigns it to $obj. It also returns the object to the code that called it. Setting permissions of private, protected and public could not have been done in PHP 4, which frustrated a lot of OOP enthusiasts. PHP 5, however, has the capability of doing it and thus fulfilling one of OOP’s main goals – encapsulation.

The first time an object of the class Database is instantiated, it will have to pass all the variables required to make a database connection. Its subsequent calls need not do that.

Both $db and $database point to the same object. Any changes made to $database will be reflected in $db also. This eliminates the use of global variables for quite a few things (especially our database class).